It should be possible to make histograms with variable-width bins. So maybe the data format would be like this:
Bin start Amount
0 5
10 8
30 12
40 6
50
Or maybe like this:
Bin start Bin end Amount
0 10 5
10 30 8
30 40 12
40 50 6
And the amount of things that fall into the bin is the *area* of the box that spans that range on the X axis. So a bin from 1-2 with 3 things in it would produce a bar as tall as a bin from 2-4 with 6 things in it.
http://ask.libreoffice.org/en/question/19816/how-do-i-make-a-histogram-with-unequal-bin-widths/